What is the material of white characters on black background in calligraphy? How to create by yourself?

Besides rubbings, there are two ways to create such calligraphy:

Bai Mo 1. In recent years, a Dege Ink Factory invented Bai Mo, which is more expensive, with a small bottle of more than 20 yuan. Just like black ink.

2. On the raw rice paper, after writing with concentrated alum water as ink, cover (brush) the back with ink. Using the principle of alum ink separation. Alum is sold in chemical stores and food stalls in large vegetable markets, that is, alum used to fry fried dough sticks. After testing the concentration of alum water several times, you can master the ratio. Water: Alum is about 8: 2.